Acquired immunodeficiency syndrome (AIDS) is a disease caused by human immunodeficiency virus (HIV) that was isolated and discovered by Montagnier L. et al., in 1983 (see Non-Patent Document 1). The origin of HIV has been considered to be the ability to infect a human, acquired due to mutation by simian immunodeficiency virus (SIV) whose natural host is a primate. HIV infects hosts not by airborne infection but through 3 main pathways: sexual transmission, blood infection, and mother-to-child transmission. The greatest cause of the sexual transmission is deemed to be contact with sexual discharges. The blood infection is caused by transfusion, wounds, needle-sharing, and so on. Therefore, infection prevention is required from an ethical standpoint. Moreover, the mother-to-child transmission is allegedly caused by contact with body fluids in the birth canal during parturition, maternal breastfeeding, virus movement through the placenta during pregnancy, and so on.
HIV, a species of retrovirus, is an adventitious virus that particularly targets and infects human CD4-positive T cells (see Non-Patent Document 1). HIV that has infected the human CD4-positive T cells is activated after a relatively long incubation period to destroy the T cells. Since T cells play an important role in the immune system, the immune capacity of the body is significantly reduced due to the destroyed T cells. As a result, the body is no longer able to exhibit sufficient resistance even to pathogens that can be eliminated easily in a normal state, falling into a chronic immunodeficient state, i.e., a state called “onset of AIDS”.
The number of HIV-infected people has already reached 33 million throughout the world, though the speed at which the number of HIV infections increases is getting slower. In 2007, the number of newly HIV-infected people was 2.4 million, and the number of deaths related to HIV was 2 million (see Non-Patent Document 2). Particularly, in Asia, which has the largest population in the world, HIV infection has rapidly been spreading. Japan is no exception. Although this is an unusual case among developed countries, the number of HIV-infected people is increasing steadily. Specifically, in 2007, the number of AIDS patients was 418, and the number of HIV-infected people was 1082, exceeding 1000 for the first time. Under the present circumstances, such increase remains to be controlled in Japan (see Non-Patent Document 3). However, HIV has been studied actively since its discovery. During years to the present, a method for diagnosing the infectious disease has been established, and overwhelming progress is also seen in therapy, compared with other infectious diseases (see Non-Patent Documents 4 and 5). By virtue of energetic research and development of therapeutic drugs, AIDS is not anymore a disease directly leading to death. However, radical therapy has not been established yet, and new problems have also emerged. Therefore, there is a demand for novel therapeutic drugs.
HIV has proteins called gp120 and gp41 necessary for binding to a target cell, on a viral membrane derived from a host cell. A matrix protein exists as a scaffold protein on the inner surface of the viral membrane and helps maintain the HIV structure. Moreover, the nucleoid of HIV has a regular dodecahedron structure surrounded by capsid proteins and internally contains the RNA genome, integrase (IN), protease (PR), and reverse transcriptase (RT) (FIG. 1). The RNA genome of HIV is approximately 9000 bp long, and the gene cluster is flanked by structures called long terminal repeats (LTRs). A dozen kinds of viral proteins encoded by these genes control complicated replication (see Non-Patent Documents 4 and 5).
A series of multiplication cycles from HIV invasion into a target cell to budding is called life cycle. This life cycle is divided into the following stages (1) to (6): (1) adsorption of the virus onto the cell membrane and membrane fusion; (2) reverse transcription of the RNA genome; (3) viral DNA integration into the host chromosome and replication; (4) processing of the constituent proteins of the virus; (5) construction of virions; and (6) the process of budding. HIV multiplies through these processes (see FIG. 2) (see Non-Patent Documents 4 to 7). Azidothymidine (AZT), the first anti-HIV drug developed in 1985, is a competitive inhibitor of reverse transcriptase. This drug inhibits HIV multiplication by inhibiting the life cycle. Since the development of this AZT, the life cycle of HIV has been revealed in more detail, and the development of anti-HIV drugs has drastically proceeded (see Non-Patent Documents 8 and 9). As a result, anti-HIV drugs as many as 15 or more kinds are currently under clinical application and have enabled highly active anti-retroviral therapy (HAART) in which anti-HIV drugs differing in mechanism of action are used in combination. By virtue of HAART, HIV is not anymore a disease directly leading to death. In addition, the quality of life (QOL) of HIV-infected people has been improved.
The mechanism by which HIV infects a target cell will be described. HIV utilizes a receptor such as CD4 or CXCR4/CXCR5 on the target cell for infecting the target cell. The envelope protein of HIV binds to such a receptor, resulting in change in the three-dimensional structure of the envelop protein, by which the viral membrane of HIV and the target cell membrane are brought close to each other to cause membrane fusion (FIG. 3). When membrane fusion is achieved, the RNA genome or viral proteins of HIV are released into the target cell to achieve infection (see Non-Patent Documents 10 to 15).
Since HIV cannot self-replicate, it can only infect a host cell to use the transcriptional and translational functions of the host cell, for multiplication. Therefore, drugs inhibiting the initial stage of HIV multiplication have received attention. Among such drugs actually under clinical application in the USA and Europe, there is a peptidic drug enfuvirtide (T-20; DP178) (see Non-Patent Documents 16 to 20). This T-20 is a partial peptide of an HIV envelope protein. This partial peptide prevents change in the three-dimensional structure of the HIV envelope protein through its binding to HIV. As a result, the membrane fusion of HIV is inhibited to prevent HIV invasion. Such peptidic HIV multiplication inhibitors are being developed actively, and the development of drugs targeting a co-receptor such as CCR5 or CXCR4 is also proceeding. However, since T-20 is a peptidic inhibitor, its problem is that oral administration is impossible (see Non-Patent Documents 21 to 30).
The RNA genome of HIV that has invaded the target cell is converted to complementary DNA by reverse transcriptase, and this DNA then forms double-stranded DNA (DNA genome) (see FIG. 2). This reverse transcriptase reaction is performed in a complex comprising enzymes and template RNA, and the DNA genome formed by this reverse transcriptase reaction is considered to then translocate into the nucleus. The complex described above is called preintegration complex (PIC). Particularly, viral protein R (vpr) contained in this complex is considered to be responsible for the translocation into the nucleus. After the translocation into the nucleus, the viral DNA genome is inserted into the chromosomal DNA of the host cell by the action of integrase (see Non-Patent Document 31).
Viral RNA, which is the principal body, is synthesized by the transcriptional function of the host cell with the inserted viral genomic DNA as a template. Moreover, viral proteins are also expressed as precursor proteins through transcription, translation, and splicing, and then, these precursor proteins are cleaved into mature proteins by protease. These mature proteins and the HIV RNA genome form a viral core, which is in turn released via the rupture of the cell membrane from within outward to thereby achieve formation of new HIV particles and release thereof from the cell.
The development of inhibitors for the enzymes essential for this life cycle of HIV, such as reverse transcriptase, integrase, and protease, has proceeded, showing a great effect on the inhibition of HIV multiplication. However, the long-term administration of drugs is required for continuously reducing virus loads in the body as much as possible. As a result, there also arise problems such as emergence of drug-resistant strains, a serious adverse reaction, and the need for expensive medical fees for treatment. Particularly, the emergence of drug-resistant viruses is a serious problem. Examples of causes of the emergence of drug-resistant viruses include long-term medication as well as mutability (HIV is highly prone to mutation due to a lack of a DNA repair mechanism such as DNA polymerase).
The base substitution rate of the HIV genome is 1 million or more times that of the mammalian genome; thus, the HIV genome changes at a rate 1 million or more times that of the mammalian genome (see Non-Patent Document 32). As a result, a point mutation is introduced in the nucleotide sequence of the HIV genome, and some strains have acquired resistance to anti-HIV drugs. In addition, cases of cross-resistance have also been reported, in which HIV that has acquired resistance to reverse transcriptase inhibitors, the main drugs in HAART, also acquires resistance to the other reverse transcriptase inhibitors. Thus, there is a demand for the development of drugs with a novel mechanism of action that can be expected to have an effect even on viruses that have acquired drug resistance (see Non-Patent Documents 4, 8, and 33).
Meanwhile, the development of HIV vaccines has been pursued for the purpose of preventing HIV. As a first step, the establishment of HIV animal models was tackled. In the late half of 1980s, the research proceeded drastically by the establishment of SIV-infected macaque monkey models. However, a basic problem, i.e., the extent to which the models reflect human HIV-1 infection, still remains to be solved (see Non-Patent Document 4). This problem holds true not only for vaccine development but for development of new drugs. Specifically, due to concerns about the insufficient reliability of HIV animal models, the possibility cannot be denied that even candidate substances of anti-HIV drugs or anti-HIV vaccine capable of exhibiting an effect on actual human patients infected with HIV are excluded from the screening of substances having an anti-HIV activity using the HIV animal models.
Another problem of the development of vaccines is that the type of immune system that should be induced, i.e., the type of immune system that should be induced for inhibiting the onset of AIDS, is unknown. Although this problem also still remains to be solved, attempts have been made recently to examine the relationship between a number of immune systems such as cell-mediated immunity and humoral immunity and the inhibition of HIV. Currently, there is also a report which concludes that the potentiation of mucosal immunity may also be effective for the inhibition of HIV (see Non-Patent Documents 34 and 35).
Under the circumstances, inactivated virus vaccines were developed as HIV vaccines using monkey models and originally reported to have a protective effect on HIV infection. However, this protective effect on HIV infection was then found to be that on human-derived antigens constituting virions. Thus, the efficacy of the vaccines was denied. Then, attenuated vaccines were shown to have efficacy, but may not be applied clinically at the present moment due to risks attributed to the mutability of HIV as described above. Thus, the development of HIV vaccines has made little progress. However, details on the mechanism by which HIV invades a target cell have been revealed. This has promoted the development of HIV vaccines based on a new point of view.
The mechanism by which HIV invades a target cell will be described. A surface protein gp120 and a transmembrane protein gp41 form a heterodimer on the surface of an HIV particle. The proteins constituting this heterodimer further form homotrimers, and a dozen such homotrimers are present on the HIV cell membrane (FIG. 4). In the course of membrane fusion of HIV, gp120 binds to CD4, a primary receptor for HIV infection, on the target cell to thereby cause its three-dimensional structure change (see Non-Patent Document 9).
Subsequently, gp120, which has become capable of binding to a co-receptor CXCR4 or CCR5, forms a gp120-CD4-co-receptor tripartite complex (see Non-Patent Documents 9 and 10). By the formation of this tripartite complex, gp41 forming the non-covalent complex with gp120 exposes its N terminus, and a membrane-inserted peptide present in gp41 anchors to the target cell membrane. After the anchoring, the N-terminal helical region N36 and C-terminal helical region C34 of gp41 bind to each other in an antiparallel manner to form a hexamer. As a result, the viral membrane of HIV and the target cell membrane are brought close to each other, causing membrane fusion (FIGS. 3 and 5) (see Non-Patent Document 10).
Based on such a mechanism of HIV invasion, some antibodies that target gp120 or gp41 and have an anti-HIV activity have been induced. Currently, b12, 2G12 and the like as antibodies against gp120, and 4E10, 2F5 and the like as antibodies against gp41 are known as those exhibiting a relatively strong anti-HIV activity (FIG. 6) (see Non-Patent Documents 36 to 43). Furthermore, Patent Document 1 describes use of antibodies against gp120 for preventing HIV infection or for inactivating a stage essential for the life cycle of HIV. However, these antibodies, albeit with an anti-HIV activity against a certain HIV strain, did not exhibit a sufficient anti-HIV activity against the other strains, probably because the HIV genome is significantly mutable. Thus, the antibodies still remain to be applied clinically.